Semiconductor device and method of fabricating same

ABSTRACT

A semiconductor device having CMOS circuits formed on a glass substrate. The CMOS circuits are composed of TFTs. Lightly doped regions are formed only in the N-channel TFTs. When P-channel TFTs are formed, the conductivity type of the lightly doped regions is converted by a boron ion implant. Each CMOS circuit consists of an N-channel TFT having the lightly doped regions and a P-channel TFT having no lightly doped regions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device consisting ofP-channel and N-channel thin-film transistors arranged on the samesubstrate and also to a method of fabricating such a semiconductordevice. More particularly, the invention relates to a CMOS circuitconfiguration composed of thin-film transistors formed on a glasssubstrate and also to a method of fabricating this CMOS circuitconfiguration.

2. Description of the Related Art

A technique for fabricating a thin-film transistor (TFT) by growing athin film of silicon on a glass substrate is known. This technique hasbeen developed to fabricate active matrix liquid crystal displays.

A liquid crystal display comprises a pair of glass substrates togetherwith a liquid crystal material held between the substrates. A largenumber of pixels are arranged in rows and columns. For each pixel, anelectric field is applied across the liquid crystal material to vary itsoptical property. Thus, an image is displayed.

In the active matrix liquid crystal display, a TFT is disposed at eachof the pixels arranged in rows and columns as described above. This TFTcontrols electric charge going into and out of the pixel electrode.

In the present technology, a peripheral driver circuit for drivinghundreds of TFTs X hundreds of TFTs arranged in the active matrix regionis composed of an IC circuit (known as a driver IC) attached to theoutside of a glass substrate by TAB (tape automated bonding) or othertechnique.

However, mounting driver IC to the outside of the glass substratecomplicates the manufacturing process. Also, the driver IC results inunevenness. This hinders wide application of the liquid crystal displayincorporated in various electronic appliances.

A technique for solving these problems consists of fabricating theperipheral driver circuit out of TFTs and integrating these TFTs withother TFTs on the glass substrate. This makes the whole system a unit.

Furthermore, the process sequence is simplified, the reliability isenhanced, and the application can be extended.

In this active matrix liquid crystal display incorporating theperipheral driver circuit as described above, CMOS circuits arenecessary to form the peripheral driver circuit. A CMOS circuit is acomplementary combination of an N-channel transistor and a P-channeltransistor, and is one of fundamental configurations of electroniccircuits. The following various methods for fabricating CMOSconfiguration out of TFTs on a glass substrate are known.

One known method is illustrated in FIGS. 4(A)-4(D). As shown in FIG.4(A), a silicon oxide film 402 acting as a buffer layer is first formedon a glass substrate 401. An active layer, 403 and 404, made ofcrystalline or amorphous silicon is formed on the silicon oxide film402. A silicon oxide film 405 serving as a gate-insulating film iscoated on the laminate. The active layer portion 403 is an island ofregion forming an active layer for an N-channel TFT. The active layerportion 404 is an island of region forming an active layer for aP-channel TFT.

After obtaining the state shown in FIG. 4(A), gate electrodes 406 and407 are fabricated out of silicide or other material (FIG. 4(B)).

Then, as shown in FIG. 4(C), phosphorus (P) ions are implanted whilemasking the other TFT region with a resist mask 408. As a result, asource region 409, a drain region 411, and a channel formation region410 for the N-channel TFT are formed by self-aligned technology.

Thereafter, as shown in FIG. 4(D), the resist mask 408 is removed. A newresist mask 412 is placed. At this time, boron (B) ions are implanted.By this manufacturing step, a source region 415, a drain region 413, anda channel formation region 414 for the P-channel TFT are formed byself-aligned technology.

In this way, the N-channel and P-channel TFTs can be formedsimultaneously on the same glass substrate. In the configuration shownin FIGS. 4(A)-4(D), the drain region 411 of the P-channel TFT isconnected with the drain region 413 of the N-channel TFT. The gateelectrodes of both TFTs are connected together. Consequently, a CMOSconfiguration is obtained.

The manufacturing steps shown in FIGS. 4(A)-4(D) are the mostfundamental processes for CMOS circuits. However, two separate masks 408and 412 used for implantation of dopant ions for imparting N-typeconductivity and P-type conductivity, respectively, are necessary. Thiscomplicates the process sequence. That is, the two resist masks 408 and412 are necessitated during the dopant ion implantation.

In order to form each resist mask, a resist material must be applied,sintered, selectively exposed, using a photomask, and selectivelyremoved for formation of the resist mask. Furthermore, where dopant ionsare implanted, using a resist as a mask, the resulting ion bombardmentmodifies the quality of the resist. This makes it difficult to removethe resist mask.

Where the manufacturing steps illustrated in FIGS. 4(A)-4(D) areadopted, it follows that two manufacturing steps for removing the resistmaterial which has been modified in quality and thus is difficult toremove are performed. This will be another factor of defects. Hence,these two steps are undesirable.

A known method of alleviating this problem is illustrated in FIGS.5(A)-5(D). As shown in FIG. 5(A), a silicon oxide film 502 is formed asa buffer layer on the glass substrate 401. An active layer, 503 and 504,of crystalline or amorphous silicon is formed on the silicon oxide film502. A silicon oxide film 505 acting as a gate-insulating film is formedover the laminate. The active layer portions 503 and 504 are islands ofregions forming active layers for N- and P-channel TFTs, respectively.Then, gate electrodes 506 and 507 of silicide or other material areformed, thus giving rise to a state shown in FIG. 5(B).

Under this condition, phosphorus (P) ions are implanted into the wholesurface. As a result, N-type regions 508, 510, 511, and 513 are formed(FIG. 5(C)). The dose of the P ions is 1×10¹⁵ to 2×10¹⁵ ions/cm². Thesurface dose is 1×10²⁰ ions/cm² or more.

Then, a resist mask 514 is placed only on selected regions forming anN-channel TFT. Boron (B) ions are implanted at a dose about 3 to 5 timesas high as the dose of the aforementioned P ions. The N-type regions 511and 513 are converted into P-type. In this way, P-channel source region515, drain region 516, and channel formation region 512 are formed byself-aligned technology.

The heavy doping described above is required because it is necessarythat the regions 515, 512, and 516 form an NIN junction. In this manner,N- and P-channel TFTs can be obtained with a fewer number of masks thanthe configuration shown in FIG. 4(A)-4(D). In the configuration shown inFIGS. 5(A)-5(D), the N-channel TFT has the source region 508, channelformation region 509, and drain region 510. The P-channel TFT has thedrain region 516, channel formation region 512, and drain region 515.Although the configuration shown in FIGS. 5(A)-5(D) has the advantagethat it can be manufactured with simplified manufacturing steps, theconfiguration has the following drawbacks.

First, dopant ions are implanted into the resist mask 514 at a quitehigh dose. This gives rise to a conspicuous modification of the qualityof the resist. This in turn often results in defective manufacturingsteps.

Secondly, the right TFT (P-channel TFT) as viewed in FIGS. 5(A)-5(D) hasthe channel formation region. The drain region adjacent to this channelformation region is a quite heavily doped region. The dose is in excessof the dose necessary for the P-channel type and sufficient fortype-conversion. Therefore, the off current near the junction betweenthe channel formation region and the drain region is negligible.

Thirdly, ions take unstraight paths, thus introducing B ions into thechannel formation region 512. As a consequence, required characteristicscannot be obtained.

Fourthly, implanting dopant ions at a high dose imposes heavy burden onthe ion implanter and on the plasma implant machine. Also, much labor isrequired to decontaminate the inside of the machine and to service themachine. In this way, various problems take place.

Fifthly, introducing dopant ions at a high dose increases the processingtime.

Sixthly, where annealing is carried out with laser light, difficultiesoccur. After the step shown in FIG. 5(D), the resist mask 514 isremoved. Then, an annealing step for activating the implanted dopantsand annealing the doped regions with laser irradiation is necessary.This method is useful where a glass substrate having poor heatproofnessis used. At this time, the regions 515 and 516 are more severelydeteriorated in crystallinity than the regions 508 and 510, because theregions 516 and 516 are more heavily doped than the regions 508 and 510.Therefore, the regions 508 and 510 differ greatly from the regions 515and 516 in dependence of light absorption coefficient on wavelength.Under this condition, the annealing effect of the laser irradiationdiffers materially between these two kinds of regions. Consequently, theleft N-channel TFT and right P-channel TFT have greatly differentcharacteristics with undesirable results.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide techniques forcircumventing the problem occurring when N- and P-channel TFTs arefabricated at the same time, i.e., increase in the number of masks, andthe problem with the steps illustrated in FIGS. 5(A)-5(D), i.e.,high-dose dopant ion implantation.

Specifically, the invention is intended to provide techniques forfabricating both N- and P-channel TFTs on a glass substrate at a lowercost and with a reduced amount of labor than heretofore and with highreliability.

It is another object of the invention to provide a method of fabricatinga CMOS circuit out of TFTs in such a way that the CMOS circuit has highcharacteristics by compensating for the differences in characteristicsbetween the N- and P-channel TFTs.

One embodiment of the present invention is a semiconductor devicecomprising an N-channel thin-film transistor and a P-channel thin-filmtransistor having source and drain regions, said N-channel and P-channelthin-film transistors being integrated on a common substrate. Lightlydoped drain (LDD) regions are formed selectively only in the N-channelthin-film transistor. The source and drain regions of the P-channelthin-film transistor are doped with P-type and N-type dopants at firstand second doses, respectively. The first dose is higher than the seconddose.

A specific example of this configuration is shown in FIG. 3(B), where anN-channel TFT (NTFT) located on the left side and a P-channel TFT (PTFT)located on the right side together form a CMOS circuit. Thisconfiguration is characterized in that a lightly doped drain (LDD)region 123 is formed selectively only in the NTFT. This LDD region 123is located between the channel formation region and the drain region.This LDD region mitigates the electric field intensity between thechannel formation region and drain region, thus reducing the off currentand suppressing deterioration. Furthermore, the LDD region increases theresistance between the source and drain so that the effective mobilityof the TFTs is reduced.

The configuration shown in FIG. 3(B) is similar to the configurationshown in FIG. 2(B) except that a dopant (P) for imparting conductivityN-type is introduced also in the right P-channel TFT. In order that theTFT finally act as a P-channel device, the source and drain regions ofthe right P-channel TFT are more heavily doped with a P-type dopant thanan N-type dopant. For this purpose, B ions are implanted during a stepillustrated in FIG. 2(C).

Where silicon is used as a semiconductor, phosphorus (P) is a typicalexample of the N-type dopant which imparts conductivity N-type. Also,where silicon is used as a semiconductor, boron (B) is a typical P-typedopant.

Where the configuration shown in FIG. 3(B) is employed, those portionsin the source and drain region of the P-channel TFT which are adjacentto the channel formation region are more lightly doped with the N-typedopant than other portions. The concentration of the P-type dopant isuniform or substantially uniform over the whole source and drainregions, because P ions imparting conductivity N-type are implanted inthe steps shown in FIGS. 1(E) and 2(B), respectively. More specifically,regions 125 and 128 are implanted with P ions twice, but regions 126 and127 are implanted with dopant ions only once. As a result, the regions126 and 127 adjacent to the channel formation regions 131 are doped withP ions more lightly than the source region 128 and drain region 125.

On the other hand, the dopant ions imparting the conductivity P-type areimplanted only once, as shown in FIG. 2(C). Therefore, the source anddrain regions are wholly doped with the P-type dopant uniformly ornearly uniformly.

Another embodiment of the invention is a semiconductor devicecomprising: an active matrix region formed on a substrate and consistingof thin-film transistors arranged in rows and columns; a peripheraldriver circuit for driving said thin-film transistors in said activematrix region, said peripheral driver circuit being formed on saidsubstrate; N-channel thin-film transistors having LDD or offset gateregions and arranged in said active matrix region; complementary N- andP-channel thin-film transistors arranged in said peripheral drivercircuit; LDD regions or offset gate regions formed selectively in theN-channel thin-film transistors arranged in said peripheral drivercircuit; and said P-channel thin-film transistors arranged in saidperipheral driver circuit having source and drain regions doped with anN-type dopant imparting conductivity N-type.

A further embodiment of the invention is a semiconductor devicecomprising: an active matrix region formed on a substrate and consistingof thin-film transistors arranged in rows and columns; a peripheraldriver circuit for driving said thin-film transistors in said activematrix region, said peripheral driver circuit being formed on saidsubstrate; P-channel thin-film transistors arranged in said activematrix region; complementary N- and P-channel thin-film transistorsarranged in said peripheral driver circuit; LDD regions or offset gateregions formed selectively in the N-channel thin-film transistorsarranged in said peripheral driver circuit; and said P-channel thin-filmtransistors arranged in said active matrix region and in said peripheraldriver circuit having source and drain regions doped with an N-typedopant imparting conductivity N-type.

A yet other embodiment of the invention is a method of fabricating asemiconductor device consisting of N-channel and P-channel thin-filmtransistors integrated on a common substrate, said method comprising thesteps of: forming gate electrodes out of a material capable of beinganodized, said gate electrodes having side surfaces; selectively forminga porous anodic oxide film on the side surfaces of said gate electrodes;implanting an N-type dopant, using said anodic oxide film as a mask, ata first dose; removing said anodic oxide film; implanting an N-typedopant, using said gate electrodes as a mask, at a second dose to formLDD regions under which said anodic oxide film existed; and implanting aP-type dopant while masking only those regions which should become theN-channel thin-film transistors.

Specific examples of the above-described structure are given below. FIG.1(D) shows a manufacturing step for forming a porous anodic oxide film,112 and 113, selectively on side surfaces of gate electrodes made of amaterial that can be anodized. FIG. 1(E) shows a step for introducing anN-type dopant, using the aforementioned anodic oxide film as a mask.FIG. 2(A) shows a state obtained after the anodic oxide film has beenremoved. FIG. 2(B) illustrates a step for introducing an N-type dopant,using the gate electrodes 11 as a mask and forming LDD regions underregions 123 where the anodic oxide film existed. FIG. 2(C) shows amanufacturing step for selectively masking those regions which shouldbecome N-channel TFTs and implanting a P-type dopant.

A method of fabricating a semiconductor device consisting of N-channeland P-channel thin-film transistors integrated on a common substrate inaccordance with the present invention comprises the steps of: forminggate electrodes out of a material capable of being anodized, said gateelectrodes having side surfaces; selectively forming a porous anodicoxide film having a thickness on the side surfaces of said gateelectrodes; implanting an N-type dopant, using said anodic oxide film asa mask; removing said anodic oxide film; implanting a P-type dopantwhile masking only regions which should become the N-channel thin-filmtransistors; and forming offset gate regions selectively in theN-channel thin-film transistors, said offset gate regions beingdetermined by the thickness of said porous anodic oxide film.

This method is characterized in that, as shown in FIGS. 6(A)-6(D),offset gate regions 613 and 614 are formed so as to have a thicknessequal to the thickness of a porous anodic oxide film 605. If a denseanodic oxide film 600 is thick, this also contributes to formation ofthe offset gate regions.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A)-1(E) are cross-sectional views of a CMOS TFT circuitaccording to the present invention, illustrating some process steps forfabricating the circuit;

FIGS. 2(A)-2(B) are cross-sectional views, illustrating process stepscarried out after the steps shown in FIGS. 1(A)-1(E);

FIGS. 3(A)-3(B) are cross-sectional views, illustrating process stepscarried out after the steps shown in FIGS. 2(A)-2(B);

FIGS. 4(A)-4(D) are cross-sectional views of a conventional CMOS TFTcircuit, illustrating a process sequence for fabricating the circuit;

FIGS. 5(A)-5(D) are cross-sectional views of a known CMOS TFT circuit,illustrating a process sequence for fabricating the circuit;

FIGS. 6(A)-6(D) are cross-sectional views of a still other CMOS TFTcircuit according to the invention, illustrating a process sequence forfabricating the circuit;

FIGS. 7(A)-7(D) are cross-sectional views of a yet other CMOS TFTcircuit according to the invention, illustrating a process sequence forfabricating the circuit;

FIG. 8 is a graph showing the dopant distribution in an active layerused in a CMOS circuit according to the invention;

FIGS. 9(A)-9(D) are cross-sectional views of an additional CMOS TFTcircuit according to the invention, illustrating a process sequence forfabricating the circuit;

FIGS. 10(A)-10(E) are cross-sectional views of a yet further CMOS TFTcircuit according to the invention, illustrating a process sequence forfabricating the circuit;

FIGS. 11(A)-11(E) are cross-sectional views of a yet additional CMOS TFTcircuit according to the invention, illustrating a process sequence forfabricating the circuit;

FIGS. 12(A)-12(E) are schematic views of various appliances utilizingelectrooptical devices according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

In the present example, a CMOS configuration is built on a glasssubstrate, using thin-film transistors (TFTs). The process sequence ofthe present example is shown in FIGS. 1(A)-1(E), 2(A)-2(D), and3(A)-3(B).

First, as shown in FIG. 1(A), a silicon oxide film 102 is formed as abuffer layer on a glass substrate 101 by sputtering or plasma CVD to athickness of about 3000 Å. The glass substrate can be made of Corning7059 glass or Corning 1737 glass. Furthermore, a transparent quartzsubstrate having high heatproofness can be used as the glass substratealthough it is expensive.

After formation of the silicon oxide film 102, a silicon film which willbecome an active layer for TFTs later is grown. In this example, anamorphous silicon film (not shown) is formed to a thickness of 500 Å byplasma CVD or LPCVD.

After forming the amorphous silicon film (not shown), it is crystallizedby laser irradiation, heat-treatment, or a combination of them. In thisway, a crystalline silicon film (not shown) is obtained.

This crystalline silicon film (not shown) is patterned to form an activelayer, 104 and 105, for N- and P-channel TFTs, respectively. Then, asilicon oxide film 103 acting as a gate-insulating film is formed to athickness of 1000 Å by plasma CVD.

In this way, the state shown in FIG. 1(A) is obtained. For simplicity,it is assumed that one pair of N-channel and P-channel TFTs is formed.

Generally, hundreds or more of pairs of N-channel and P-channel TFTs areformed on the same glass substrate.

After deriving the condition shown in FIG. 1(A), an aluminum film 106which will form gate electrodes later is formed by sputtering orelectron-beam evaporation, as shown in FIG. 1(B). In order to suppressgeneration of hillocks and whiskers, the aluminum film contains 0.2% byweight of scandium. Hillocks are small, elevated areas. Whiskers areneedle-like protrusions. Both kinds of protrusions are produced byabnormal growth of aluminum. Hillocks and whiskers cause electricalshorts and crosstalks between adjacent conductive interconnects andbetween adjacent metallization levels.

Besides aluminum, a metal such as tantalum capable of being anodized canbe used. After growing the aluminum film 106, an anodization process iscarried out within an electrolytic solution, using the aluminum film 106as an anode. As a result, a thin, dense, anodic oxide film 107 isformed.

In this example, the electrolytic solution is prepared by neutralizingethylene glycol solution containing 3% tartaric acid with ammonia. Thisanodization method permits formation of a dense anodic oxide film. Thefilm thickness can be controlled by the applied voltage.

In this example, the thickness of the anodic oxide film 107 is about 100Å. This anodic oxide film 107 acts to promote adhesion to a resist maskformed later. In this way, a state shown in FIG. 1(B) is obtained.

Then, a resist mask, 108 and 109, is formed. Using this resist mask, 108and 109, the aluminum film 106 and the overlying anodic oxide film 107are patterned, thus obtaining a state shown in FIG. 1(C).

Subsequently, using 3% aqueous solution of oxalic acid, an anodizationprocess is performed while employing an aluminum film pattern, 110 and1111, left in the solution as an anode.

During this anodization process, anodization selectively progresses onthe side surfaces of the left aluminum film pattern, 110 and 111,because the dense anodic oxide film and the resist mask, 108 and 109,remain on the top surface of the aluminum film pattern, 110 and 111.

As a result of this anodization, a porous anodic oxide film is formed.This porous film can be grown up to several micrometers. It is to benoted that the aforementioned anodic oxide film can be grown up to about3000 Å. Consequently, the anodic oxide film, or more correctly anodicoxide, indicated by 112 and 113, is formed. In this example, theanodization is caused to proceed until a film thickness of 7000 Å isreached. This film thickness of the anodic oxide will determine thelength of lightly doped regions formed later. Empirically, it is desiredto grow the porous anodic oxide film to 6000-8000 Å. In this way, astate shown in FIG. 1(D) is obtained.

Under this condition, gate electrodes 11 and 12 are defined. Afterobtaining the state shown in FIG. 1(D), the resist mask, 108 and 109, isremoved.

Then, an anodization process is carried out, using an electrolyticsolution prepared by neutralizing ethylene glycol solution containing 3%tartaric acid with ammonia. In this process, the electrolytic solutionenters the porous anodic film, 112 and 113. As a result, a dense anodicoxide film, 114 and 115, is formed, as shown in FIG. 1(E).

This dense anodic oxide film, 114 and 115, has a thickness of 600 Å. Theremaining portions of the previously formed dense anodic oxide film 107merge with the anodic oxide film, 114 and 115.

Under the condition shown in FIG. 1(E), phosphorus (P) ions areimplanted as an N-type dopant imparting conductivity N-type into thewhole surface by plasma doping. This implant is performed at a high doseof 0.2 to 5×10¹⁵/cm², preferably 1 to 2×10¹⁵/cm². This doping isconveniently referred to as heavy doping. As a result of the step shownin FIG. 1(E), regions 116, 117, 118, and 119 heavily doped with P ionsare formed.

Then, the porous anodic oxide film, 112 and 113, is removed, usingaluminum mixed acid. In this way, a state shown in FIG. 2(A) isobtained. Then, P ions are again implanted, as shown in FIG. 2(B) at alow dose of 0.1 to 5×10¹⁴/cm², preferably 0.3 to 1×10¹⁴/cm². In this ionimplantation, the P concentration at the surface is less than2×10¹⁹/cm³. That is, the dose of the P ions introduced by the step shownin FIG. 2(B) is lower than the dose of the implantation performed by thestep shown in FIG. 1(E). This is conveniently referred to as lightdoping. Consequently, lightly doped regions 121, 123, 126, and 127 arecreated. Regions 120, 124, 125, and 128 are more heavily doped with Pions.

In this manufacturing step, the region 120 becomes a source region foran N-channel TFT. The regions 121 and 123 are lightly doped regions. Theregion 124 is a drain region. The region 123 becomes a so-called lightlydoped drain (LDD) region.

Then, as shown in FIG. 2(C), a resist mask 129 that covers the N-channelTFT is placed. Under the condition shown in FIG. 2(C), boron (B) ionsare implanted at a dose of 0.2 to 10×10¹⁵/cm², preferably about 1 to2×10¹⁵/cm². This dose can be on the same order as the dose used in thestep shown in FIG. 1(E). In this step, the N-type regions 125, 126, 127,and 128 are converted into P-type. In this manner, source region 130 anddrain region 132 are formed for a P-channel TFT. A region 131 remainsundoped and forms a channel formation region.

Before an implant of B ions is made, the regions 126 and 127 shown inFIG. 2(B) are lightly doped with P ions. Accordingly, the B implanteasily converts the conductivity type. Especially, the NI junction withthe channel formation region 131 is readily converted into a PIjunction. That is, the required junction can be easily created.

Therefore, the conductivity type of the regions 126 and 127 can beconverted into the opposite type at a dose comparable to the dose of theP ion implant carried out in the step FIG. 1(E). As a result, P-typedoped regions 130 and 132 can be formed.

Since the dose can be made lower than in the prior art techniqueillustrated in FIGS. 5(A)-5(D), modification of quality of the resistmask due to dopant implantation can be suppressed.

After the completion of the step shown in FIG. 2(C), the resist mask 129is removed, thus obtaining a state shown in FIG. 2(D). Under thiscondition, laser irradiation is performed to activate the implanteddopant and to anneal the doped regions. At this time, the source/drainregions 120 and 124 of the N-channel TFT does not differ greatly incrystallinity from the source/drain regions 130 and 132 of the P-channelTFT, because no quite heavy doping is done in the step of FIG. 2(C),unlike the prior art process shown in FIG. 5(D). Hence, the annealingeffect can compensate for the difference in crystallinity. As a result,the difference in characrteristics between the obtained N- and P-typeTFTs can be compensated for.

After obtaining the state shown in FIG. 2(D), an interlayer dielectricfilm 133 is formed by plasma CVD, as shown in FIG. 3(A). The interlayerdielectric film 133 is made of silicon nitride and has a thickness of4000 Å.

Then, contact holes are created. A source electrode 134 and a drainelectrode 135 are formed for the N-channel TFT (NTFT). At the same time,a source electrode 137 and a drain electrode 136 for the P-channel TFT(PTFT) are formed. At this time, the laminate is patterned in such a waythat the drain electrode 135 of the N-channel TFT is connected with thedrain electrode 136 of the P-channel TFT and that the gate electrodes ofthe two TFTs are connected together. Thus, a CMOS structure iscompleted.

In the CMOS structure shown in FIG. 3(B), the lightly doped regions 121and 123 are disposed in the N-channel TFT. These lightly doped regions121 and 123 act to reduce the leakage current. Furthermore, they protectthe TFTs from hot carrier deterioration. In addition, they increase theresistance between the source and drain and lower the mobility of theNTFT.

Generally, in the case of the CMOS structure shown in FIG. 3(B),differences in characteristics between the N- and P-channel TFTs presentproblems. Where a crystalline silicon film is used as in the presentexample, the mobility of the N-channel TFT reaches 100 to 150 V·s/cm².However, the mobility of the P-channel TFT is only 30 to 80 V·s/cm².Furthermore, the N-channel TFT suffers from hot carrier deterioration,though the P-channel TFT does not have such a drawback. Also, CMOScircuits generally do not require low off current characteristics.

Under these circumstances, the lightly doped regions 121 and 123 aredisposed in the N-type TFT. This yields the following advantages. Themobility of the N-type TFT of the CMOS configuration is reduced. Also,the TFT is prevented from being deteriorated. In this way, the balancein characteristics between the N-type and P-type devices is improved. Asa consequence, the characteristics of the CMOS circuit can be improved.

In the ion implantation steps shown in FIGS. 1(E), 1(B), and 1(C), it isimportant that the active layer be masked with the silicon oxide film103 forming the gate-insulating film. Under this condition, if dopantions are implanted, roughening or contamination of the active layersurface can be suppressed. This contributes greatly to improvements ofthe production yield and reliability of the final product.

EXAMPLE 2

The present example relates to a CMOS structure composed of TFTsincluding N-channel TFTs. The structure is characterized in that offsetgate regions are formed only in the N-channel TFTs. The offset gateregions are similar in functions with lightly doped regions typified byLDD regions. In particular, the offset gate regions act to reduce theleakage current. Also, they increase the resistance between the sourceand drain and thus lower the mobility of the TFT. Furthermore, theyprotect the N-channel TFT from hot carrier deterioration.

A process sequence for fabricating the CMOS structure of the presentexample is illustrated in FIGS. 6(A)-6(D). First, a state shown in FIG.6(A) is obtained by performing steps similar to the steps shown in FIGS.1(A)-1(E). In FIG. 6(A), a dense anodic oxide film 600 is formed aroundeach gate electrode to a thickness of 600 Å. A porous anodic oxide film,605 and 606, has a thickness of 2000 to 4000 Å. This film thicknessalmost determines the dimensions of offset gate regions formed later.Strictly, the thickness of the dense anodic oxide film 600 locatedinside the porous anodic oxide film affects the dimensions of the offsetgate regions. However, as already described in Example 1, the thicknessis about 600 Å and so the presence of the inner anodic oxide film 600 isneglected here.

Under this condition, P ions are implanted at a heavy dose of 0.2 to5×10¹⁵/cm², preferably about 1 to 2×10¹⁵/cm², by ion implantationtechniques. As a result, regions 601-604 are heavily doped with P ions.

Then, the porous anodic oxide film, 605 and 606, is removed, thusobtaining a state shown in FIG. 6(B). Under this condition, regions 607and 608 are undoped with P ions.

Thereafter, as shown in FIG. 6(C), a resist mask is placed on portionswhich become the regions of the N-channel TFTs. This is followed by aboron (B) ion implant. The dose is 0.2 to 5×10¹⁵/cm², preferably 1 to2×10¹⁵/cm². The implant is made by a plasma doping process. As a resultof this step, regions 610 and 612 are doped P-type.

Those regions which are located just under the gate electrode andadjacent to the source/drain regions are not implanted with the P ionsin the step of FIG. 6(A). This undoped region is located immediatelyunder the porous anodic oxide film portion 606. Since this undopedregion is a substantially intrinsic region, it can be easily convertedinto P-type by B ion implant shown in FIG. 6(C). Hence, the dose of theB ions in this step can be reduced to a minimum requisite value. In thisway, drain region 610, channel formation region 611, and source region612 of the P-channel TFT can be formed by self-aligned technology.

Then, the resist mask 609 is removed, thus obtaining a state shown inFIG. 6(D). Under this condition, indicated by 601 and 602 are the sourceand drain regions of the N-channel TFT. Indicated by 614 is the channelformation region.

Offset gate regions 613 and 615 are applied with no electric field fromthe gate electrodes. Also, the offset gate regions 613 and 615 do notact as source/drain regions. These offset gate regions serve to mitigatethe field intensity between the source/drain (especially the drainregion) and the channel formation region. On the other hand, theP-channel TFT contains no offset gate region.

This configuration substantially reduces the mobility of the N-channelTFT and suppresses deterioration of the characteristics, as previouslydescribed in Example 1. In consequence, the balance in characteristicsbetween the N-channel and P-channel TFTs of the CMOS structure isimproved.

EXAMPLE 3

The present example is an improvement of the lightly doped regionstructure formed in the N-channel TFT described in Example 1. A lightlydoped region is mainly placed between a channel formation region and adrain region and acts to mitigate the electric field strength betweenboth regions.

Generally, active layers of TFTs have amorphous, microcrystalline, andpolycrystalline states and, therefore, the junction structure adjacentto the channel tends to be weak. This gives rise to various problems,including variations in characteristics among TFTs, aging of thecharacteristics, and deterioration of the reliability.

Accordingly, in the present example, the concentration distribution inthe lightly doped region disposed between the channel formation regionand the drain (source) region is controlled, thus solving the foregoingproblems.

In the lightly doped region of the present example, the dopantconcentration gradually decreases from the drain and source regiontoward the channel formation. If the junction structure is weak, thisstructure can suppress the various problems with the TFT, i.e.,variations in characteristics among individual devices, aging of thecharacteristics, and deterioration of the reliability.

FIGS. 7(A)-7(D) show a CMOS structure composed of TFTs of the presentexample. First, the manufacturing steps of Example 1 are performed untilthe state of FIG. 1(E) is reached, i.e., prior to dopant ionimplantation.

Then, dopant ions such as P ions are implanted, as shown in FIG. 7(A),under appropriate conditions so that the P ions may be implanted underthe porous anodic oxide film, 701 and 702, after following unstraightpaths. As a result, heavily doped regions 703, 707, 708, and 712 areformed. In each of lightly doped regions 704, 706, 709, and 711, thedopant concentration varies continuously or in a stepwise manner.Channel formation regions 705 and 710 are left undoped.

The P ions are implanted into regions becoming source and drain at adose of 0.2 to 5×10¹⁵/cm², preferably 1 to 2×10¹⁵/cm². An example of Pion concentration distribution obtained by such a dopant implant isshown in FIG. 8. This concentration distribution can be controlled bythe ion implant conditions used in the step shown in FIG. 7(A). The ionsgiving the concentration distribution shown in FIG. 8 follow unstraightpaths because the insulating film overlying each doped region is made toassume a positive potential with respect to the gate electrode byelectrification.

In the configuration shown in FIG. 8, the conductivity type can be madeto vary continuously or in a stepwise fashion and so the field strengthapplied to the junction can be mitigated. This enhances the reliabilityof the device.

After the implantation of the P ions as shown in FIG. 7(A), the porousanodic oxide film, 701 and 702, is removed, thus obtaining a state shownin FIG. 7(B). Then, a resist mask 713 is placed on the N-channel TFT.Subsequently, boron (B) ions are implanted at a dose of 0.2 to5×10¹⁵/cm², preferably 1 to 2×10¹⁵/cm² (FIG. 7(C)). As a result of thismanufacturing step, the N-type regions 708, 709, 711, and 712 areconverted into P-type. Because the regions 709 and 711 are lightly dopedalso in this step, and because the dopant concentration decreases towardthe channel, their conductivity type can be readily converted.Consequently, a P-channel TFT having a drain region 714, a channelformation region 710, and a source region 715 can be derived (FIG.7(D)).

Also, an N-channel TFT having the source region 703, the lightly dopedregions 704, 706, the channel formation region 705, and the drain region707 is obtained. The drain regions of both TFTs are connected together,and their gate electrodes are connected together. Thus, a CMOS structureis obtained.

In the present example, the presence of the lightly doped regionssubstantially lowers the mobility of the N-channel TFT and suppressesdeterioration of the N-channel type. Furthermore, the balance incharacteristics between the P-channel and N-channel types can becorrected. In consequence, a CMOS circuit having high characteristicscan be fabricated.

EXAMPLE 4

The present invention relates to a structure in which the channel of theN-channel TFT is lightly doped P-type to control the threshold value ofthe N-channel TFT.

The process sequence of the present example is similar to the processsequence of Example 1 shown in FIGS. 1(A)-1(E), 2(A)-2(D), and 3(A)-3(B)except that a trace amount of diborane (B₂H₆) is added to the gaseousraw material during growth of an amorphous silicon film which is astarting film for the active layer, 104 and 105. The amount of the addeddiborane may be determined, taking account of the threshold valuecharacteristics of the obtained TFT. More specifically, the amount ofthe added diborane is so adjusted that the dose of boron finallyremaining in the channel formation region is about 1×10¹⁷ to 5×10¹⁷/Cm².

EXAMPLE 5

In Example 4, the channel formation region of the N-channel TFT islightly doped P-type in order to control the threshold value of theN-channel TFT. In Example 4, however, the threshold value of theP-channel TFT cannot be controlled at will.

Accordingly, in the present example, under the state shown in FIG. 1(A)or prior to this state, i.e., before the gate-insulating film 103 isformed, dopant ions are selectively implanted into the active layer, 104and 105. For example, prior to the state shown in FIG. 1(A), i.e.,before the gate-insulating film 103 is formed, the active layer portion105 is masked and B ions are implanted into the active layer portion 104at a desired dose. As a result, the active layer 104 is lightly dopedP-type.

Then, P ions are implanted into the active layer 105 while masking theactive layer portion 104. As a result, the active layer portion 105 islightly doped N-type. In this way, the threshold values of the N-channeland P-channel TFTs can be controlled independently.

After dopant ions are implanted into the active layer as in the presentexample, annealing is preferably done by heat-treatment or laserirradiation. This annealing is effective in activating the implanteddopant ions and repairing the damage caused by the ion implantation.

EXAMPLE 6

The present example is similar to the configuration of Example 1 exceptthat offset gate regions are formed in addition to the lightly dopedregions 121 and 123 (FIG. 2(B)). The offset gate regions protect thedevices from hot carrier deterioration and reduce the off current. Also,the offset gate regions substantially lower the mobility by increasingthe resistance between the source and drain. That is, the offset gateregions are similar in functions with lightly doped regions typified byLDD regions.

The process sequence of the present example is illustrated in FIGS.9(A)-9(D) and similar to the process sequence of Example 1 (FIGS.1(A)-1(E), 2(A)-2(D), and 3(A)-3(B)) unless stated otherwise. It is tobe noted that like components are indicated by like reference numeralsin various figures. The present example is characterized in that a denseanodic oxide film, 901 and 902, formed over the surface of the gateelectrode as shown in FIG. 9(A), has an increased film thickness of 2000to 2500 Å.

The film thickness may be increased further, but the voltage appliedduring anodization exceeds 300 V. In this case, reproducibility andsafety will present problems.

This dense anodic oxide film is formed essentially similarly to themethod of Example 1 except that the applied voltage is varied accordingto the film thickness. As the applied voltage is increased, thethickness of the anodic oxide film increases.

After forming the thick, dense anodic oxide film, 901 and 902, describedabove (FIG. 9(A)), P ions are implanted under the same conditions as inExample 1 (FIG. 9(B)). A source region 120, a drain region 124, and achannel formation region 122 for an N-channel TFT are formed byself-aligned technology. Also, lightly doped regions 121 and 123 areformed. In this example, the lightly doped region 123 is an LDD region.A pair of offset gate regions 903 are formed on opposite sides of thechannel. The offset gate regions 903 act neither as channels nor assource/drain regions. The dimensions of the offset gate regions aresubstantially determined by the thickness of the dense anodic oxide film901 formed on the surface of the gate electrode in the step shown inFIG. 9(A).

After the end of the step shown in FIG. 9(B), a resist mask 129 isplaced and B ions are implanted under the same conditions as in Example1 (FIG. 9(C)). As a result of this step, a drain region 130, a sourceregion 132, and a channel formation region 131 for a P-channel TFT areformed by self-aligned technology. An offset gate region 904 whosethickness is equal to the anodic oxide film 902 is formed.

Then, the resist mask 129 is removed, thus obtaining a state shown inFIG. 9(D). Thereafter, annealing making use of laser irradiation iscarried out.

In the present example, the left N-channel TFT has both a lightly dopedregion and an offset gate region. On the other hand, the right P-channelTFT has no lightly doped region but has an offset gate region.

If the thickness of the dense anodic oxide film, 901 and 902, isreduced, then the function of the offset gate regions diminishes.Finally, the same configuration as the configuration of Example 1 isobtained.

No clear minimum width of the offset gate regions exist at which theyfunction satisfactorily. That is, no clear minimum thickness of theanodic oxide film, 901 and 902, exists. Accordingly, in theconfiguration of Example 1, an offset gate region can exist between thesource region and the channel formation region, and another offset gateregion can exist between the drain region and the channel formationregion, irrespective of whether the offset gate regions functionsatisfactorily.

EXAMPLE 7

The present example relates to a structure in which an active matrixregion and a peripheral driver circuit for driving the active matrixregion are integrated on a glass substrate.

An integrated active matrix liquid crystal display has a pair ofsubstrates. One of the substrates is made of glass or quartz. The activematrix region has pixels arranged in rows and columns. At least oneswitching TFT is located at each pixel. The peripheral driver circuit isdisposed around the active matrix region. All of these circuits areintegrated on the aforementioned glass or quartz substrate.

Where the present invention is applied to this active matrix liquidcrystal display, N-channel TFTs having low off current characteristicsare arranged in the pixel regions. The peripheral circuit can becomposed of CMOS circuits having high characteristics.

In particular, the peripheral circuit is made of the CMOS configurationshown in FIGS. 1(A)-1(E), 2(A)-2(D), and 3(A)-3(B)). The N-channel TFTshown to the left of each of these figures and similar N-channel TFTsare arranged in the active matrix region.

The TFTs disposed in the active matrix region are required to retainelectric charge in their pixel electrodes for a given time and so it isdesired to make their off current as small as possible. Therefore, TFTsequipped with the lightly doped regions 121 and 123 shown to the left ofFIG. 3(B) are best suited for this purpose.

On the other hand, the peripheral driver circuit is frequently made ofCMOS circuits. In order to enhance the characteristics of the CMOScircuits, it is necessary that the N- and P-channel TFTs forming eachCMOS circuit have quite uniform characteristics. The CMOS configurationshown in FIGS. 1(A)-1(E), 2(A)-2(D), and 3(A)-3(B) is best suited forthis purpose. The active matrix liquid crystal display can beconstructed by integrating these CMOS circuits each having its preferredcharacteristics.

In the present example, each N-channel TFT has lightly doped region(LDD) regions. Alternatively, the N-channel TFT may have offset gateregions in the same way as in Example 2. Furthermore, TFTs arranged inthe active matrix region can be of the P-type.

EXAMPLE 8

In the present example, LDD regions or offset gate regions are formedwithout utilizing anodization.

The process sequence of the present example is shown in FIGS.10(A)-10(E). First, a silicon oxide film 1002 is formed as a bufferlayer on a glass substrate 1001. An active layer, 1003 and 1004, made ofcrystalline silicon is formed. The active layer portion 1003 will becomethe active layer of an N-channel TFT, while the active layer portion1004 will become the active layer of a P-channel TFT.

Then, a silicon oxide film 1005 acting as a gate-insulating film isgrown. Thereafter, a silicon film consisting of microcrystallitesheavily doped with P or B is formed. Using a resist mask, 1008 and 1009,the film is patterned to form a film pattern, indicated by 1006 and1007. Gate electrodes will be formed, based on this film pattern. Thus,a state shown in FIG. 10(A) is obtained.

Then, an isotropic dry etching process is carried out to form a pattern,indicated by 1010 and 1011 in FIG. 10(B). Under the condition shown inFIG. 10(C), P ions are implanted at a high dose similarly to otherexamples described already. Thus, regions 1012, 1014, 1015, and 1017 areheavily doped with P ions. Regions 1013 and 1016 are left undoped.

Then, as shown in FIG. 10(D), the resist mask, 1008 and 1009, isremoved. P ions are again implanted at a low dose similarly to the otherexamples previously described. As a result, regions 1018, 1020, 1021,and 1023 are lightly doped with P ions.

Then, as shown in FIG. 10(E), B ions are implanted while masking theN-channel TFT portion by a resist mask 1024. This implantation is madeunder such conditions that the N-type regions 1015, 1021, 1017, and 1023are converted into P-type. Since the regions 1021 and 1023 are lightlydoped with P ions, regions 1025 and 1026 can be converted into P-typewithout performing a high-dose boron implant, as previously described inother examples. The regions 1025 and 1026 are required as source/drainregions of the P-channel TFT.

After the ion implantation shown in FIG. 10(E), laser irradiation iscarried out to active the implanted dopant ions and to anneal out thedamage caused by the implantation. Manufacturing steps similar to thesteps of other examples are performed. In this way, N-channel andP-channel TFTs are completed.

The N-channel TFT has the source region 1012, the lightly doped region1018, the channel formation region 1019, the lightly doped region 1020(LDD region), and the drain region 1014. On the other hand the P-channelTFT has the source region 1025, the channel formation region 1022, andthe drain region 1026. In the step shown in FIG. 10(D) if a low-doseimplant is not effected, P ions are not implanted into the regions 1018and 1020 at a low dose. In this case, these regions can be used asoffset gate regions.

EXAMPLE 9

In the present example, LDD regions or offset gate regions are formedwithout utilizing anodization. The process sequence of the presentexample is illustrated in FIGS. 11(A)-11(E). First, a silicon oxide film1102 is formed as a buffer layer on a glass substrate 1101. An activelayer, 1103 and 1104, made of crystalline silicon is formed. The activelayer portion 1103 will become the active layer of an N-channel TFT,while the active layer portion 1104 will become the active layer of aP-channel TFT.

Then, a silicon oxide film 1105 acting as a gate-insulating film isgrown. Thereafter, a silicon film consisting of microcrystallitesheavily doped with P or B is formed. Using a resist mask (not shown),the film is patterned to form a film pattern, indicated by 1106 and1107. Gate electrodes will be formed from this film pattern.

Thereafter, a silicon nitride film 1108 is formed, thus obtaining astate shown in FIG. 11(A). This silicon nitride film 1108 is etched by adry etching process having vertical anisotropy. At this time, theetching conditions are appropriately selected in such a way thatsubstantially triangular residues, 1109 and 1110, of silicon nitride arecreated. In this way, a state shown in FIG. 11(B) is obtained.

Then, in a manufacturing step shown in FIG. 11(C), P ions are implantedat a heavy dose. As a result, regions 1111, 1113, 1114, and 1116 areheavily doped with P ions. Regions 1112 and 1115 are left undoped.Subsequently, the silicon nitride film, 1109 and 1110, is removed. Undera condition shown in FIG. 11(D), P ions are implanted at a light dose.As a result, regions 1117, 1119, 1120, and 1122 become lightly dopedregions (N-regions). Regions 1118 and 1121 become channel formationregions.

Thereafter, as shown in FIG. 11(E), B ions are implanted while maskingthe N-channel TFT portion by a resist mask 1123. This implantation isperformed in such conditions that N-type regions 1114, 1116, 1120, and1122 are converted into P-type. Since the regions 1120 and 1022 arelightly doped with P ions, regions 1124 and 1125 can be converted intoP-type without performing a high-dose boron implant, as previouslydescribed in other examples. The regions 1124 and 1125 are required assource/drain regions of the P-channel TFT.

After the end of the implantation step shown in FIG. 11(E), laserirradiation is carried out to activate the implanted dopant ions and toanneal out the damage caused by the implantation. Manufacturing stepssimilar to the steps of other examples are performed. In this way,N-channel and P-channel TFTs are completed.

The N-channel TFT has a source region 1111, a lightly doped region 1112,a channel formation region 1118, a lightly doped region 1119 (LDDregion), and a drain region 1113. On the other hand, the P-channel TFThas a source region 1124, a channel formation region 1121, and a drainregion 1125. In the step shown in FIG. 11(D), if a low-dose implant isnot effected, P ions are not implanted into the regions 1117 and 1119 ata low dose. In this case, these regions can be used as offset gateregions.

EXAMPLE 10

The present invention can be applied to an electrooptical device havingthe active matrix construction. Especially, the invention can be appliedwith great utility to a peripheral driver circuit incorporated in anintegral electrooptical device. Besides peripheral driver circuits, atleast parts of memories treating image signals and various kinds ofsignals and information-treating circuits can be constructed, making useof the present invention.

Specifically, the invention can be applied to various kinds of circuitsintegrated on one substrate, in addition to an active matrix circuit.Examples of the above-described electrooptical device include liquidcrystal displays, electroluminescent devices, and electrochromicdisplays. They find practical applications in TV cameras, personalcomputers, car navigational systems, TV projection systems, videocameras, and portable intelligent terminals. Some of them are nextdescribed briefly by referring to FIGS. 12(A)-12(E).

Referring to FIG. 12(A), there is shown a TV camera. The body of thiscamera is indicated by numeral 2001. This TV camera comprises the body2001, a camera section 2002, a display unit 2003, and operation switches2004. The display unit 2003 is used as a viewfinder. This apparatusshown in FIG. 12(A) can be employed as a portable intelligent terminal.

Referring next to FIG. 12(B), there is shown a personal computer. Thebody of this computer is indicated by numeral 2101. This personalcomputer comprises the body 2101, a cover 2102, a keyboard 2103, and adisplay unit 2104. The display unit 2104 is used as a monitor andrequired to have diagonal dimensions as large as more than ten inches.

Referring next to FIG. 12(C), there is shown a car navigational system.The body of this system is indicated by numeral 2301. The body 2301includes a display unit 2302 and operation switches 2303. Thenavigational system further includes an antenna 2304. The display unit2302 is used as a monitor.

Referring next to FIG. 12(D), there is shown a TV projection system. Thebody of this system is indicated by numeral 2204. This body includes alight source 2402, a display unit 2403, mirrors 2404, 2405, and a screen2406. An image displayed on the display unit 2403 is projected onto thescreen 2406 and so the display unit 2403 is required to have highresolution.

Referring next to FIG. 12(E), there is shown a video camera. The body ofthis camera is indicated by numeral 2501. This body includes a displayunit 2502, an eyepiece 2503, operation switches 2504, and a tape holder2505. An image picked up and displayed on the display unit 2502 can beviewed on a real-time basis through the eyepiece 2503. Hence, the usercan take pictures while watching the image.

The present invention yields the following advantages.

(1) Only one implant mask is necessary to fabricate a CMOS structure andso the manufacturing processing can be simplified.

(2) Lightly doped regions are formed only in the N-channel TFT.Therefore, a CMOS structure having well balanced characteristics can bemanufactured.

(3) Since no quite heavy doping is necessary, the resist can beprevented from being modified in quality.

(4) The conductivity type can be easily converted, because regionsadjacent to the channel are intrinsic or lightly doped regions.

(5) Since the active layer is coated with a silicon oxide film,contamination and surface roughening can be circumvented.

1. (Cancelled)
 2. A TV system comprising: operation switches; and adisplay unit, the display unit comprising: a thin film transistorincluding a source region, drain region, a channel region formed betweenthe source and drain regions, and an LDD region formed between thechannel region and at least one of the source and drain regions, whereinthe LDD region includes first and second regions and an impurityconcentration of the first region is different from that of the secondregion.
 3. A TV system according to claim 2, wherein the thin filmtransistor is an N-channel thin film transistor.
 4. A TV systemcomprising: operation switches; and a display unit, the display unitcomprising: a thin film transistor including a source region, drainregion, a channel region formed between the source and drain regions,and an LDD region formed between the channel region and at least one ofthe source and drain regions, wherein an impurity concentration changescontinuously in the LDD region.
 5. A TV system according to claim 4,wherein the thin film transistor is an N-channel thin film transistor.6. A camera comprising: an eyepiece; operation switches; and a displayunit, the display unit comprising: a thin film transistor including asource region, drain region, a channel region formed between the sourceand drain regions, and an LDD region formed between the channel regionand at least one of the source and drain regions, wherein the LDD regionincludes first and second regions and an impurity concentration of thefirst region is different from that of the second region.
 7. A TV systemaccording to claim 6, wherein the thin film transistor is an N-channelthin film transistor.
 8. A camera comprising: an eyepiece; operationswitches; and a display unit, the display unit comprising: a thin filmtransistor including a source region, drain region, a channel regionformed between the source and drain regions, and an LDD region formedbetween the channel region and at least one of the source and drainregions, wherein an impurity concentration changes continuously in theLDD region.
 9. A TV system according to claim 8, wherein the thin filmtransistor is an N-channel thin film transistor.
 10. A computercomprising: keyboard; and a display unit, the display unit comprising: athin film transistor including a source region, drain region, a channelregion formed between the source and drain regions, and an LDD regionformed between the channel region and at least one of the source anddrain regions, wherein the LDD region includes first and second regionsand an impurity concentration of the first region is different from thatof the second region.
 11. A TV system according to claim 10, wherein thethin film transistor is an N-channel thin film transistor.
 12. Acomputer comprising: keyboard; and a display unit, the display unitcomprising: a thin film transistor including a source region, drainregion, a channel region formed between the source and drain regions,and an LDD region formed between the channel region and at least one ofthe source and drain regions, wherein an impurity concentration changescontinuously in the LDD region.
 13. A TV system according to claim 12,wherein the thin film transistor is an N-channel thin film transistor.14. A portable intelligent terminal comprising: a display unit, thedisplay unit comprising: a thin film transistor including a sourceregion, drain region, a channel region formed between the source anddrain regions, and an LDD region formed between the channel region andat least one of the source and drain regions, wherein the LDD regionincludes first and second regions and an impurity concentration of thefirst region is different from that of the second region.
 15. A portableintelligent terminal according to claim 14, wherein the thin filmtransistor is an N-channel thin film transistor.
 16. A portableintelligent terminal comprising: a display unit, the display unitcomprising: a thin film transistor including a source region, drainregion, a channel region formed between the source and drain regions,and an LDD region formed between the channel region and at least one ofthe source and drain regions, wherein an impurity concentration changescontinuously in the LDD region.
 17. A portable intelligent terminalaccording to claim 16, wherein the thin film transistor is an N-channelthin film transistor.